The present application relates to a battery pack and a control method thereof for detecting an abnormality in the battery pack.
In recent years, mobile electronic devices, such as a notebook PC (Personal Computer), a mobile phone, and a PDA (Personal Digital Assistant), have been widespread. As the power supply of the mobile electronic devices, a lithium-ion secondary battery having such advantages as high voltage, high energy density, and lightweight has been widely used.
A battery pack of a secondary battery normally includes a fuse called chemical fuse as a protective device which operates in the event of overcharge or overcurrent to the secondary battery. The chemical fuse is configured to include a fuse and a heater resistor. Due to the heat generated by current flowing into the heater resistor, the fuse is fusion-cut.
The battery pack as described above is configured such that, in an abnormal state such as overcharge and overcurrent, current is applied to the heater resistor of the chemical fuse to fusion-cut the fuse and stop the charge or discharge of the secondary battery. As used throughout the present application, the term fusion-cut or fusion-cutting refers to a fuse being heated and melted to cut off a charge or discharge current of a secondary battery.
With reference to FIG. 4, a configuration example of a related art battery pack 100 will now be described. A positive terminal 111 and a negative terminal 112 of the battery pack 100 are connected to a positive terminal and a negative terminal of an external electronic device or charger, respectively, to discharge or charge the battery pack 100. A secondary battery 102 is a lithium-ion secondary battery, for example, and includes one or a plurality of battery cells 115 connected in series and/or parallel.
An AFE (Analog Front End) 103 measures the respective voltages of the battery cells 115 of the secondary battery 102, and measures the magnitude and direction of current by using a current detection resistor 107. Then, the AFE 103 supplies the measured values to an MPU (Micro Processing Unit) 104. Further, on the basis of a command from the MPU 104 described later, the AFE 103 transmits a control signal to a switch circuit 105 to prevent overcharge or overdischarge.
The switch circuit 105 is configured to include a charge control FET (Field Effect Transistor) 116a and a discharge control FET 117a. If the battery voltage reaches an overcharge detection voltage, the switch circuit 105 is controlled to turn OFF the charge control FET 116a, to thereby prevent the flow of charge current. After the turn-OFF of the charge control FET 116a, only the discharge can occur via a parasitic diode 116b. 
Further, if the battery voltage falls to an overdischarge detection voltage, the switch circuit 105 is controlled to turn OFF the discharge control FET 117a, to thereby prevent the flow of discharge current. After the turn-OFF of the discharge control FET 117a, only the charge can occur via a parasitic diode 117b. 
A chemical fuse 106 is configured to include fuses 118 and a heater resistor 119, and is connected in series to the secondary battery 102. If overcurrent to the secondary battery 102 is detected, for example, the overcurrent directly flows into the fuses 118. As a result, the fuses 118 generate heat and are fusion-cut.
Further, if overcharge to the secondary battery 102 is detected, for example, a FET 108 is turned ON on the basis of the control by the MPU 104 described later. As a result, a path is formed through which current flows from the secondary battery 102 into the FET 108 via the fuses 118 and the heater resistor 119. Then, the current flows into the heater resistor 119, and the heater resistor 119 generates heat. Due to the heat from the heater resistor 119, the fuses 118 are fusion-cut.
On the basis of the voltage value of the secondary battery 102 and the current value supplied by the AFE 103, the MPU 104 supplies the AFE 103 with a command for controlling the switch circuit 105, if the voltage of any one of the battery cells 115 of the secondary battery 102 reaches the overcharge detection voltage or falls to or below the overdischarge detection voltage. Further, upon detection of an abnormality, the MPU 104 controls a gate signal of the FET 108 to turn ON the FET 108, and performs a fusion-cutting process on the chemical fuse 106.
Further, the MPU 104 includes a counter 110 to count the time from the start of the fusion-cutting process which starts upon turn-ON of the FET 108. The counter 110 is previously set with a time-out time, i.e., the completion time of the counting operation. At the set time-out time, the counting operation is completed.
If the fuses 118 are fusion-cut, the MPU 104 transmits a fusion-cut alarm, which indicates that the fuses 118 have been fusion-cut, to the electronic device connected to the battery pack 100, via communication terminals 113 and 114.
Subsequently, with reference to the flowchart of FIG. 5, description will be made of the flow of processing performed when an abnormality of the secondary battery 102 is detected in the related art battery pack 100. The following processing is assumed to be performed under the control of the MPU 104. Further, the following processing is assumed to be cyclically performed every predetermined time, e.g., every second.
At Step S101, upon detection of an abnormality such as overcharge or overcurrent to the secondary battery 102, for which the fusion-cutting of the fuses 118 is necessary, it is determined whether or not a fusion-cut condition of the fuses 118 has been met, or whether or not the fusion-cut alarm has been set. At the initial stage, the fusion-cut alarm is reset. If the fusion-cut condition has been met, or if the fusion-cut alarm has been set, it is determined that an abnormality of the secondary battery 102 has been detected, and the processing proceeds to Step S102. At Step S102, in accordance with the control signal transmitted from the AFE 103 on the basis of the command from the MPU 104, the charge control FET 116a and the discharge control FET 117a are brought into the OFF state.
At Step S103, whether or not the fusion-cut alarm has been set is determined. If it is determined that the fusion-cut alarm has been set, the processing proceeds to Step S104 to start a count-up operation of the counter 110.
Meanwhile, if it is determined at Step S103 that the fusion-cut alarm has not been set, the processing proceeds to Step S105. At Step S105, a fuse fusion-cutting process is started. Thus, the FET 108 is turned ON to apply current to the heater resistor 119 of the chemical fuse 106, and the count value of the counter 110 is initialized to start the counting operation. Further, the fusion-cut alarm is set and transmitted to the electronic device connected to the battery pack 100, via the communication terminals 113 and 114.
At Step S106, it is determined whether or not the count value of the counter 110 has reached the previously set time-out time. If the count value has reached the time-out time, the processing proceeds to Step S107. Then, the fuse fusion-cutting process is completed, and the counting operation by the counter 110 is completed. If the count value has not reached the time-out time at Step S106, the processing returns to Step S101.
Meanwhile, if the fusion-cut condition has not been met and the fusion-cut alarm has not been set at Step S101, the state of the secondary battery 102 is determined to be normal, and the processing returns to Step S101.
As described above, in the related art battery pack, the fuses are fusion-cut upon detection of an abnormality. Accordingly, it is possible to cut off the charge or discharge current of the secondary battery, and thus to prevent a dangerous state.
If the battery voltage or the capacity of the secondary battery is low in the battery pack as described above, however, it is difficult to apply sufficient current to the heater resistor. As a result, there arises a situation in which the temperature of the heat generated by the heater resistor does not reach a fuse fusion-cut temperature and thus the fuses fail to be fusion-cut.
Further, if the spatial distance between the fuses and the case of the battery pack is short in such a situation, the case of the battery pack may be fused. In general, the heat-resistance temperature of resin or the like used as the material of the case of the battery pack is approximately 80° C., which is lower than the fuse fusion-cut temperature, which is approximately 130° C. For this reason, there arises an issue of the dangerous state described above, which is caused by the continuation of a state in which the temperature of the fuses is higher than the heat-resistance temperature of the case of the battery pack and is lower than the fuse fusion-cut temperature.
Therefore, to address the above-described issues, Japanese Unexamined Patent Application Publication No. 2007-215310, for example, describes a control method of a battery pack, which performs a charging operation when the battery voltage or the battery capacity of a secondary battery is low, to thereby apply sufficient current to a heater resistor.